1. Field of the Invention
The present invention concerns new compounds with high latent isocyanate content and to methods for producing the same.
2. Description of the Prior Art
The production of masked isocyanates, also called blocked isocyanates is known and is described in Houben-Weyl, Methoden der Organische Chemie XIV/2, pp. 61-70. Known blocking agents are tertiary alcohols, phenols, acetoacetic ester, malonic ester, acetyl acetone, phthalamide, imidazol, hydrogen chloride, hydrogen cyanide and .epsilon.-caprolactam. In practice, the use of monophenols and .epsilon.-caprolactam has been preferred for this purpose.
These masked isocyanates have the property of reacting like isocyanates at elevated temperatures by cleavage of the blocking groups. The more acidic the H-atom of the blocking or masking group, the easier the cleavage. Such blocked isocyanates are described in DT-OS No. 21 66 423. Also, terminally blocked isocyanates, which additionally contain uretdion groups, have been described in Dt-OS No. 25 02 934.
Blocked isocyanates are advantageous because they may be admixed with isocyanate reactable materials, such as hydroxylgroup containing polymers to form a stable mixture. When a reaction is desired, as in the production of polyurethanes from a polyisocyanate and a polyol, heat is applied to the mixture causing unblocking of the isocyanate, which permits the desired reaction to occur. Such compositions find utility as coating methods.
The most important drawback to using these blocking agents in a number of applications is the relatively high cleavage temperature, which for most phenols with aliphatic polyisocyanates, is at least 190.degree. C. and with aromatic polyisocyanates about 30.degree. C. lower. For this reason it has been proposed in the past to use thiophenols for which this temperature is much lower. The disadvantage of the thiophenols, however, lies in their exceedingly unpleasant odor.
It has also been proposed to admix 1-2 percent of a catalyst with the blocked isocyanate. This expedient can reduce cleavage temperatures to 125.degree.-140.degree. C. It has been found, however, that the introduction of such catalysts can cause the durability of the unhardened coating material to be adversely affected. The cleavage temperature of aliphatic .epsilon.-caprolactam-blocked polyisocyanates generally lies at about 180.degree.-190.degree. C.
Accordingly, a need continues to exist for a blocked polyisocyanate which is characterized by the fact that the blocking group can be removed at relatively low temperatures.